Lithium ion batteries are secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material. They operate by the transfer of lithium ions between the anode and the cathode, and they are not to be confused with lithium batteries, which are characterised by containing metallic lithium. Lithium ion batteries are currently the most commonly used type of rechargeable battery and typically the anode comprises an insertion material, for example carbon in the form of coke or graphite. An electroactive couple is formed using a cathode that comprises a lithium-containing insertion material. Typical lithium-containing insertion materials are lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) and lithium manganese oxide (LiMn2O4). In its initial condition, this type of cell is uncharged, therefore to deliver electrochemical energy the cell must be charged to transfer lithium to the anode from the lithium-containing cathode. Upon discharge, the lithium ions are transferred from the anode back to the cathode. Subsequent charging and discharging operations transfers the lithium ions back and forth between the cathode and the anode over the life of the battery. A review of the recent developments and likely advantages of lithium rechargeable batteries is provided by Tsutomu Ohzuku and Ralph Brodd in Journal of Power Sources 2007.06.154.
Unfortunately, lithium cobalt oxide is a relatively expensive material and the nickel compounds are difficult to synthesize. Not only that, cathodes made from lithium cobalt oxide and lithium nickel oxide suffer from the disadvantage that the charge capacity of a cell is significantly less than its theoretical capacity. The reason for this is that less than 1 atomic unit of lithium engages in the electrochemical reaction. Moreover, the initial capacity is reduced during the initial charging operation and still further reduced during each charging cycle. Prior art U.S. Pat. No. 4,828,834 attempts to control capacity loss through the use of a cathode mainly composed of LiMn2O4. U.S. Pat. No. 5,910,382 on the other hand, describes another approach using lithium-mixed metal materials such as LiMPO4 where M is at least one first row transition metal. Preferred compounds include LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4 and mixed transition metal compounds such as Li1-2xFe1-xTixPO4 or Li1-2xFe1-xMnxPO4 where 0<x<1.
The use of lithium ion rechargeable batteries is limited by the prohibitive cost of providing the lithium electrode material, particularly in the case of lithium cobalt oxide. Consequently, current commercialisation is restricted to premium applications such as portable computers and mobile telephones. However, it would be highly desirable to gain access to wider markets, for example the powering of electric vehicles and work has been ongoing in recent years to produce materials that maintain the high performance of lithium ion batteries, but which at the same time, are much cheaper to produce. To achieve this goal, it has been suggested, for example in JP Kokai No 10208782 and Solid State Ionics 117 (1999) 273-276), that sulfides may be used in place of oxides, as cathode materials. Although the use of many sulfides achieves less voltage measured against lithium of the corresponding oxides, the capacity of some sulfide-based cathodes, measured in milliampere hours per gram, can be as much as about 3 times greater. Based on this, some sulfide-based cathodes achieve an overall advantage of about 1.5 times in terms of cathode energy density for batteries measured against a lithium metal anode, as compared against their oxide counterparts, and this makes the use of these sulfides a very attractive proposition. For example, in the case of lithium iron sulfide a theoretical capacity of 400 mAhg−1 may be obtained with an average operating voltage of 2.2V versus a lithium metal anode.
Thus, lithium-containing transition metal sulfides will be a convenient substitute material for the lithium metal oxides described above, with lithium iron sulfide being already described in the patent literature, for example in U.S. Pat. No. 7,018,603, to be a useful cathode material in secondary cells. The commercialisation of lithium-containing transition metal sulfides will depend largely on their cost of production. Taking lithium iron sulfide as a specific example, the conventional process for making this material is via a solid state reaction in which lithium sulfide (Li2S) and ferrous sulfide (FeS), are intimately mixed together and heated under an inert atmosphere at a temperature of about 800° C. The starting materials ferrous sulfide (FeS) and iron disulfide (FeS2) are relatively inexpensive as they are found as naturally occurring materials, and are dug out of the ground. However, a notable disadvantage of the reaction process is that the other starting material, Li2S, is not only expensive but also highly moisture sensitive. The latter problem in particular has obvious implications for the complexity, and therefore the cost, of storing and handling the starting material, especially for large-scale commercial production. In addition, the kinetics of this reaction are reported in U.S. Pat. No. 7,018,603 to be very slow and it can apparently take up to one month to complete the reaction, thus this route is believed to be highly unfavourable in terms of energy costs and not commercially viable for the production of electrode materials.
As an alternative route for making lithium-containing transition metal sulfides, U.S. Pat. No. 7,018,603 discloses reacting a transition metal sulfide such as FeS with lithium sulfide in a reaction medium comprising molten salt or a mixture of molten salts at high temperature (temperatures of 450° C. to 700° C. are exemplified). The preferred molten salts are lithium halides. Whilst this reaction proceeds at a good rate there are still several issues that make it less than ideal. Firstly, the fact it uses Li2S as a starting material leads to the handling and storage problems described above. Secondly, it is very difficult to separate the reaction medium (molten lithium halide used in 1.5 molar excess) from the desired reaction product other then by solvent extraction, and this type of extraction is expensive. Further, even after rigorous purification as much as 8% of the reaction medium salt is still present in the reaction product, and this level of impurity is detrimental to the charge capacity per gram of lithium iron sulfide.
Given the expense of producing lithium sulfide, it is highly desirable to find further alternative routes for its production, which is simple, energy efficient and produces a clean product.